Repairing And Microstructuring Of Silicon Surfaces With Nanosecond Laser

Silicon exists in the form of single-crystal silicon and amorphous silicon.Singlecrystal silicon is employed extensively in the semiconductor,micro-machine,and optoelectronic industries.Amorphous silicon has been gaining increasing attention in the fabrication of solar cells.In order to improve the energy absorption ratio of silicon film surfaces,micro-structures should be fabricated on them.Silicon is a hard and brittle material,contact processing will cause surface and subsurface damage,such as microcracks,dislocations and phase transitions etc.,which seriously affects the mechanical and optical properties of silicon devices.Therefore,it is quite necessary to explore a more efficient processing method.As one of the most flexible tools for multi-scale structure patterning,laser has the advantages of high precision,strong controllability and selective irradiation etc.,and gradually applied in ultra-precision and micro-nano manufacturing.As a non-contact processing method,laser irradiation technology can effectively avoid the generation of surface and subsurface damage.Compared with femtosecond laser and picosecond laser,nanosecond laser is a long pulse laser.Using its instantaneous thermal effect,on the one hand,surface defects can be repaired;on the other hand,the formation of microstructures can be induced.Thus,nanosecond laser irradiation technology is an ideal processing approach for single crystal silicon and amorphous silicon.However,the flow of the surface molten layer caused by thermal effects is uncontrollable and multi-directional.Meanwhile,due to the influence of thermal effects,the material in the active region will produce bulges,cracks,remelting etc.,which seriously limit the processing accuracy of nanosecond lasers.In view of the above background,this paper focuses on the response characteristics of nanosecond laser processing silicon surface,and mainly carries out the following researches:1.The characteristics of nanosecond laser processing and the influence of various process parameters on the processing effect were analyzed.Then,for the laser irradiation of silicon surfaces,a two-dimensional heat transfer transient model and a three-dimensional heat transfer transient model was established,respectively.The simulation analysis of the temperature changes on the silicon surfaces irradiated by single-pulse and multi-pulse lasers was carried out.2.Aiming at the surface defects such as grooves,notches and cracks generated during slicing of single-crystal silicon,a two-step laser irradiation approach was proposed.First,the repairable defect depth under different laser parameters was forecasted by finite element simulation.Thereafter,by using two-step laser irradiation,the surface defects were repaired while obtaining nanoscale surface roughness based on Marangoni convection and surface tension effects.Moreover,by optimizing process parameters,the periodic tool marks produced by single point diamond turning could be effectively repaired through laser irradiation,and the "grating effect" was eliminated.The surface roughness of the material was reduced from Ra 10 nm to Ra 1 nm.3.Through analyze morphology changes of the silicon film surface of different initial conditions produced by nanosecond laser irradiation,entirely different surface topographies were observed.The effects of laser irradiation parameters,such as repetition frequency,beam overlap ratio,and scanning speed,on the surface topographies were investigated,followed by the characterization of surface roughness,elemental composition,stress state,and crystallinity of the irradiated surfaces.In addition,the temperature field of the material under different laser fluences irradiation was simulated.Based on the above analysis,the physical mechanisms for the different phenomena on the two surfaces were discussed.4.The influences of peak power density and scanning distance on nanosecond laser-induced micro-structures on amorphous silicon surfaces were investigated via single-and multi-line irradiation experiments.The shape,basement diameter,height,and material composition of the micro-structures were analyzed under various conditions.The correlation between the height of the micro-structure and the laser peak power density and the scanning distance was obtained,and an approach for fabricating micro-structures with different periodicities in a large area was proposed.Meanwhile,the study found that three structures with different shapes and sizes evolved on the surface of the silicon film under three typical peak power densities.In response to this phenomenon,finite element simulation of laser irradiation and energy dispersive X-ray spectroscopy analysis of the micro-structured surfaces were performed.On this basis,the formation mechanism of micro-structures of multiple scales was proposed.5.Laser irradiation experiments were carried out on reaction boned silicon carbide and amorphous silicon surfaces based on single-crystal silicon and quartz,and the effects of different substrate materials on the micro-structure were discussed.When reaction boned silicon carbide was irradiated,the bulk structures were formed on the surface due to the difference in ablation thresholds between silicon carbide and silicon.Under higher beam overlap ratios,the surface of the silicon film with single crystal silicon as the base forms woven structures of morphological characteristics are different from protrusion and pillar,which interlace on the surface of the material.When quartz was used as substrates,micro-pillar structures were formed on the surface of the amorphous silicon.Based on this phenomenon,the formation mechanism of micropillars was analyzed.The study demonstrate that laser irradiation technology can not only repair various surface defects of single-crystal silicon,but also achieve nanometric surface roughness.Meanwhile,periodic micro-structures can be fabricated on the surface of amorphous silicon by adjusting the laser fluence and changing the processing approach.These researches found,on the one hand,provides an effective new method for ultraprecision manufacturing of single-crystal silicon.On the other hand,it enhances researchers' fundamental understanding of the formation of micro-structures and offers theoretical and technical basis for the industrialization of solar cells.